Microscope illumination optical system

ABSTRACT

An illumination optical system for a microscope is provided wherein the illumination state can be successively changed between Koehler illumination and critical illumination while ensuring one or more conditions are satisfied so that the image of the light source illuminates an appropriately large region of the field of view of the microscope during critical illumination. Preferably, no cemented lens elements are used so that degradation of the cement caused by ultraviolet light sources is avoided, thereby enabling an appropriate illumination type to be provided at will and without degradation of the optical components of the illumination optical system over time.

This is a divisional application of allowed U.S. Application Ser. No.10/115,144 filed Apr. 4, 2002 now U.S. Pat. No. 6,836,358, the benefitof priority of which is hereby claimed under 35 U.S.C. § 120.

BACKGROUND OF THE INVENTION

In general, Koehler illumination optical systems are widely used as theillumination optical system for microscopes because it makes thebrightness of the field of view uniform FIG. 7 shows the basicconstruction in the case where a Koehler illumination optical system isused with a microscope to illuminate an object using light that isincident substantially normal to a support surface 9 which supports anobject of interest. In FIG. 7, a light source 1 emits illumination lightwhich is gathered by a collector lens 2 and passes this light via afirst illumination lens 3 an aperture stop 4 a field stop 5 a secondillumination lens 6, a dichroic mirror 7 (which may instead be a beamsplitter), and an object lens 8 to a sample surface 9. The collectorlens 2 the first illumination lens 3 the aperture stop 4 the field stop5 and the second illumination lens 6 form the illumination lens system.

As compared to the viewing area obtainable by using one's eyes to view amagnified image of a sample by looking into a microscope, only a morenarrow viewing area in the vicinity of the center of the field of viewcan be observed using, for example, a CCD to capture the image and tooutput the image data to a display, such as a T.V. monitor. For anillumination optical system for a microscope that is more suitable forT.V. viewing, what is termed ‘critical illumination’ is often used. Thistype of illumination optical system projects an image of a light sourceonto a center portion of a sample surface to be viewed, brightlyilluminating the center portion of the field of view.

As shown in FIG. 8A, a magnified image of a sample positioned at samplesurface 9 may be viewed in reflected light using the objective lens 8and an imaging lens (tube lens). In this case, the reflectingillumination optical system is formed of the light source 1, theillumination lens system (composed of the components 2-6, discussedpreviously), the dichroic mirror 7, and the objective lens 8. As shownin FIG. 8B, a magnified image of a sample positioned at a sample surface9 may be viewed in transmitted light using an objective lens and animaging lens. In this case, the transmitting illumination optical systemis formed of the light source 1, the illumination lens system, thedichroic mirror 7, and a condenser lens. Because the components of theillumination lens system are the same as in the Koehler illuminationoptical system shown in FIG. 7, the components themselves will not beseparately discussed with regard to the illumination optical systemsshown in FIGS. 8A and 8B. In FIGS. 8A and 8B, the illumination lightthat is incident onto the sample surface 9 that supports a sample is notcollimated, as in the case of Koehler illumination illustrated in FIG.7. Instead, a focused image of the light source is projected onto thesample surface 9. This is termed critical illumination.

The light source used for a microscope is generally a halogen lamp or anarc-discharge lamp, both of which have a small region that emits a highintensity light beam. For example, this region for an arc-discharge lamptypically measures about 0.6 mm in diameter. If an arc-discharge lamp isused in Koehler illumination, the projected image of the lamp isgenerally too small to fill the pupil of the objective lens of themicroscope, and thus the brightness of the field of view decreases.Therefore, there has been a problem using a Koehler illumination opticalsystem as shown in FIG. 7 in that both the required illumination field(i.e. the area of illumination) and the required brightness ofillumination cannot be simultaneously provided. However, using acritical-type illumination optical system as illustrated in FIG. 8A forviewing the sample using reflected light, or as illustrated in FIG. 8Bfor viewing the sample using transmitted light, also has problems inthat only the center portion of the field of view can be brightlyilluminated. Thus, neither type of illumination optical system is fullysatisfactory in terms of practical usage.

An illumination optical system for a microscope is provided with a lightsource, an illumination lens system which gathers light from the lightsource and directs the light along a light path, and an object opticalsystem which converges the light beams from the illumination lens systemso as to illuminate a sample for observation with a microscope usingeither transmitted or reflected light. The object optical system isformed of the objective lens 8, as shown in FIG. 8A, in the case ofusing reflected light to view a sample. In the case of using transmittedlight to view a sample, the object optical system is instead formed of acondenser lens, as illustrated in FIG. 8B.

The microscope optical system includes an objective lens and an imaginglens that, together, form a magnified image of the sample. In the caseof using reflected light to view a sample, the objective lens 8 (FIG.8A), serves a dual role. It not only serves as part of the illuminationoptical system to converge the light from the illumination lens systemthat is to illuminate the sample, it also serves as the objective lensof the microscope to gather the light reflected by the sample and todirect the light to the imaging lens of the microscope.

BRIEF SUMMARY OF THE INVENTION

In the present invention, either Koehler or critical illumination can beprovided to a sample, at will, and the change in illumination type isachieved by changing the spacings of one or more optical components orby moving the position of the light source. The object of the presentinvention is to provide an illumination optical system for a microscopewherein the illumination state can be changed at will between Koehlerillumination and critical illumination, and vice-versa, so as to providean illumination field and brightness which is most appropriate for agiven observation, and which uses optical components that will not beharmed by exposure to ultraviolet light.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given below and the accompanying drawings, whichare given by way of illustration only and thus are not limitative of thepresent invention, wherein:

FIG. 1 shows the basic optical component configuration of anillumination optical system for a microscope according to a firstembodiment of the invention;

FIG. 2 shows the basic optical component configuration of anillumination optical system for a microscope according to a secondembodiment of the invention;

FIG. 3 shows the basic optical component configuration of anillumination optical system for a microscope according to a thirdembodiment of the invention;

FIGS. 4A and 4B show the basic optical component configuration of theillumination optical system of a third embodiment of the invention. FIG.4B is a partial view of some of the optical components shown in FIG. 4Aas seen by looking in the direction of the arrow X in FIG. 4A;

FIGS. 5A and 5B show the basic optical component configuration of theillumination optical system of a fourth embodiment of the invention.FIG. 5B is a partial view of some of the optical components shown inFIG. 5A as seen by looking in the direction of the arrow X in FIG. 5A;

FIGS. 6A and 6B show the basic optical component configuration of theillumination optical system of a fifth embodiment of the invention. FIG.6B is a partial view of some of the optical components shown in FIG. 6Aas seen by looking in the direction of the arrow X in FIG. 6A;

FIG. 7 is a diagram that shows a prior art optical componentconstruction in the case of using a Koehler illumination optical systemto illuminate a surface which supports a sample, with the illuminationbeing normal to the surface;

FIG. 8A shows a prior art optical component configuration in the case ofusing critical illumination to view a sample in reflected light; and

FIG. 8B shows a prior art optical component configuration in the case ofusing critical illumination to view a sample in transmitted light.

DETAILED DESCRIPTION

In the present invention, an illumination optical system for amicroscope is provided with a light source, an illumination lens systemwhich gathers light from the light source and directs the light to asample, and an object optical system which is arranged between theillumination lens system and the sample so as to illuminate the samplefor observation with a microscope using either transmitted or reflectedlight. The microscope includes an objective lens and an imaging lensthat, together, form a magnified image of the sample. The microscopealso includes a condenser lens. When the sample is viewed usingtransmitted light, the object optical system is a condenser lens. Whenthe sample is viewed using reflected light, the object optical system isthe objective lens.

The illumination optical system of the present invention ischaracterized by the fact that at least one of the lens components ofthe illumination lens system or the light source is movable along theoptical axis in order to illuminate the sample with light that iscollimated during Koehler illumination, and that forms an image of thelight source on the sample during critical illumination, with thefollowing Condition (1) being satisfied:0.15<|(a·f ₂)/(b·f ₁)|<0.5  Condition (1)where

a is the diameter of the light emission region of the light source,

b is the diameter of the field of view of the microscope,

f₁ is the focal length of the illumination lens system, and

f₂ is the focal length of the object optical system.

Condition (1) controls the ratio of the size of the image of the arcfrom an arc-discharge lamp which is projected onto the sample surfacedivided by the diameter of the field of view of the microscope. Whenthis ratio falls below the lower limit of Condition (1), the arc imagebecomes too small relative to the field of view of the microscope,resulting in the bright field illumination being too small uponswitching to critical illumination. In this case, the focal length ofthe illumination optical system becomes longer, the magnification of thelight source when using Koehler illumination becomes less, and the fieldof view becomes darker. If the above-mentioned ratio is 0.5, thediameter of the projected arc image will be one-half the diameter of thefield of view of the microscope. If the upper limit of Condition (1) isnot satisfied, the arc image that is projected onto the sample surfacewhen using critical illumination becomes larger and the area ofillumination becomes broader. However, the focal length of theillumination optical system becomes shorter, and the magnification ofthe light source when using Koehler illumination becomes too large. As aresult, the numerical aperture (NA) of the cone of light rays thatenters the object lens becomes smaller and it becomes difficult toprovide uniform illumination when using Koehler illumination.

According to the present invention, the use of cemented lenses in thoseportions of the illumination optical system near an image surface of thelight source is avoided by using only non-cemented lens elements nearimage surfaces of the light source. This avoids degradation in thetransmissivity of the cement used in a cemented lens that occurs overtime when such a lens is exposed to intense ultraviolet light, as occursnear an image surface of the light source when an arc light source, suchas a mercury lamp, is used as the illumination light source.

As illustrated in FIG. 3, a field stop may be arranged within theillumination optical system at a position (surface 13) that is conjugateto the sample position. Further, a lens group A and a lens group B, eachof which has positive refractive power, may be arranged between thefield stop and the pupil of the objective lens, with the followingCondition (2) being satisfied:f_(A) <f _(B)  Condition (2)where

f_(A) is the focal length of the lens group A, and

f_(B) is the focal length of the lens group B.

The illumination optical system shown in FIG. 3 is suitable for aninvert type microscope. In the invert type microscope, it is preferablefor easy operation that the field stop is positioned away from theoptical axis of the objective lens. But if the field stop is positionedaway from the optical axis of the objective lens, the focal length (acomposite focal length) of the lens group A and the lens group B becomeslong. As a result, it is difficult to form an image of the field stop ina proper size. However, if the Condition (2) is satisfied, it ispossible to prevent the composite focal length of the lens group A andthe lens group B from becoming long.

With this construction, the focal lengths of the lens groups A and Bthat project the field stop to the sample can be shortened, the stopmechanism can be used in common with other illumination opticalsystem(s), thereby saving production costs.

According to one aspect of the invention, when a microscope is used toobserve a sample the microscope includes an objective lens and animaging lens, the objective lens collimating light reflected from thesample and directing it to the imaging lens. The illumination opticalsystem of the present invention is characterized by the fact that atleast one of the lens components of the illumination lens system or thelight source is moveable along the optical axis in order to change theillumination state from Koehler illumination to critical illumination,and the following Condition (3) is satisfied:0.15<|0.0273·f ₃ /f ₁|<0.5  Condition (3)where

f₃ is the focal length of the imaging lens of the microscope, and

f₁ is the focal length of the illumination lens system.

Furthermore, according to the present invention, a lens group thatcorrects for chromatic aberrations may be arranged between the positionof an image of the light source which is formed using Koehlerillumination, and the position of an image of the light source which isformed using critical illumination. It is necessary that chromaticaberration of the optical system which is positioned between the fieldstop and object optical system is corrected. If chromatic aberration isnot corrected, the image of the field stop which is projected on thesample surface has a different size and different position in eachcolor. Preferably, rather than using a cemented lens to correct forchromatic aberrations as in prior art illumination optical systems, thepresent invention instead merely uses two lens elements of oppositerefractive power and different dispersions. In order to avoid thedegradation in optical transmission that arises over time when acemented lens is exposed to ultraviolet radiation, the two lens elementsin the present invention are made to have their adjacent surfaces eithertouching or spaced in air 1 mm or less (i.e., no cement is used).

According to another aspect of the present invention, a field stop ispositioned at a location within the illumination lens system that isconjugate to the sample position, and a lens group that corrects forchromatic aberrations is positioned between the field stop and theobject optical system. Further, the following Condition (4) ispreferably satisfied by the lens group that corrects for chromaticaberrations:15<υp−υn<50  Condition (4)where

υp is the Abbe number of the positive lens element of the lens groupthat corrects for chromatic aberrations, and

υn is the Abbe number of the negative lens element of the lens groupthat corrects for chromatic aberrations.

If the lower limit of Condition (4) is not satisfied, chromaticaberrations of the illumination optical system cannot be sufficientlycorrected. If the upper limit of Condition (4) is not satisfied, atleast one of the two glass types of the positive and negative lenselements of the lens group that corrects for chromatic aberrations willtend to absorb too much ultraviolet light, thereby diminishing theamount of ultraviolet light that is transmitted. Therefore, fluorescenceobservation of the sample becomes difficult.

Various embodiments for the present invention will now be explained indetail with reference to the drawings.

Embodiment 1

FIG. 1 shows the basic lens element configuration of an illuminationlens system for a microscope according to a first embodiment of theinvention. This embodiment is constructed so that the position of thelight source, such as an arc-discharge lamp positioned at the far rightside of the figure, may be moved along the optical axis in the directionof the arrow by changing the surface spacing D1 from 16.7702 to 14.6986(i.e., by moving the light source a distance of 2.0716 mm toward thesample) in order to change the illumination state from Koehlerillumination to critical illumination.

Table 1 below lists the surface numbers #, in order, beginning with thelight source, the radius of curvature R (in mm) of each optical elementsurface, the on-axis spacing D (in mm) between surfaces, as well as theindex of refraction N (at 488 run) and the Abbe number υ_(d) (at the dline) for each lens element of Embodiment 1. In the middle,portion ofthe table are listed the values for D1 for providing Koehler versuscritical illumination to the sample. In the bottom portion of the tableis listed the focal length of the illumination lens system of thisembodiment. In this embodiment, when the illumination state is Koehlerillumination, an image of the light source is formed at the aperturestop (R14). When the illumination state is critical illumination, animage of the light source is formed at the field stop (R15).

TABLE 1 # R D N υ_(d) 1 (Light source) D1 (variable) 2 19.5520 8.93001.49268 70.21 3 13.3870 0.3000 4 −185.1800 6.0000 1.49268 70.21 534.4940 1.0000 6 −29.5740 8.0700 1.49268 70.21 7 83.2100 10.6300 818.8800 2.5000 1.60711 39.29 9 153.8230 26.8762 10 −19.7002 8.00001.52236 64.14 11 −24.8724 50.0968 12 −29.3628 5.0000 1.52236 64.14 13−93.9452 19.5807 14 ∞ (aperture stop) 28.9600 15 ∞ (field stop) 29.090016 98.5500 3.5000 1.60711 39.29 17 −20.0000 13.5000 1.49268 70.23 1843.0250 0.4000 19 −43.2730 7.0000 1.49268 70.23 20 64.2210 121.5000 21(OBJECTIVE pupil) for Koehler for critical illumination illuminationValue of D1: 16.7702 14.6986 Focal length of illumination lens systemfor both Koehler and critical illumination = −12.27

In this case, the object optical system is an objective lens. Themagnification of the objective lens is 10× (ten times), so that thefocal length f₂ is 18.0 mm. The light source is an arc light sourcewhich is used in the microscope generally. The power of the arc lightsource is 100 watts, and the diameter “a” of the light emission regionis 0.6 mm. An image height is 22 mm so that the diameter “b” of thefield of view is 2.2 mm, and the ratio set forth above in Condition (1)has a value of 0.4, which satisfies Condition (1). With regard toCondition (3), if the focal length of the imaging lens of the microscopeis 180 mm, the absolute value of ((0.0273·180)/(−12.27)) equals 0.4.Thus, Embodiment 1 also satisfies Condition (3) above.

Embodiment 2

FIG. 2 shows the basic lens element configuration of an illuminationlens system for a microscope according to a second embodiment of theinvention. In this embodiment, a single lens element (indicated withcross-hatching in the figure) is designed so as to be movable along theoptical axis in the direction indicated by the arrow when changing fromKoehler illumination to critical illumination.

Table 2 below lists the surface numbers #, in order, beginning with thelight source, the radius of curvature R (in mm) of each optical elementsurface, the on-axis spacing D (in mm) between surfaces, as well as theindex of refraction N (at 488 nm) and the Abbe number υ_(d) (at the dline) for each lens element of Embodiment 2. In the middle portion ofthe table are listed the values for D9 and D11 for providing Koehlerversus critical illumination to the sample. In the bottom portion of thetable is listed the focal length of the illumination lens system of thisembodiment. In this embodiment, when the illumination state is Koehlerillumination, an image of the light source is formed at the field stop(R14). When the illumination state is critical illumination, an image ofthe light source is formed at the pupil of the objective lens (R19).

TABLE 2 # R D N υ_(d) 1 (Light source) 15.5000 2 19.5520 8.9300 1.4926870.21 3 13.3870 0.3000 4 −185.1800 6.0000 1.49268 70.21 5 34.4940 1.00006 −29.5740 8.0700 1.49268 70.21 7 83.2100 10.6300 8 18.8800 2.50001.60711 39.29 9 153.8230 D9 (variable) 10 −78.8735 8.0000 1.52236 64.1411 34.4880 D11 (variable) 12 52.6303 2.2000 1.60711 39.29 13 −24.643715.8893 14 ∞ (field stop) 103.1446 15 −100.9788 3.3600 1.60711 39.21 16−37.5516 0.0217 17 −37.7820 7.0800 1.49267 70.23 18 75.0696 101.5000 19(OBJECTIVE pupil) for Koehler for critical illumination illuminationValue of D9: 27.0986  9.9969 Value of D11 21.9008 39.0025 Focal lengthof illumination lens system of this embodiment for Koehler illumination= −30.00 mm

As is evident from studying Table 2 above, the value of D9 is changedfrom 27.0986 to 9.9969 and the value of D11 is changed from 21.9008 to39.0025 when changing from Koehler illumination to criticalillumination. This is accomplished by moving the single lens element adistance of 17.1017 mm.

In this case, the object optical system is an objective lens. Themagnification of the objective lens is 10× (ten times), so that thefocal length f₂ is 18.0 mm. The light source is an arc light sourcewhich is used in the microscope generally. The power of the arc lightsource is 100 watts, and the diameter “a” of the light emission regionis 0.6 mm. An image height is 22 mm so that the diameter “b” of thefield of view is 2.2 mm, and the ratio set forth above in Condition (1)has a value of 0.164, which satisfies Condition (1). Also, thisembodiment satisfies Condition (3).

As is illustrated in FIG. 2, this lens group is arranged between thepupil of the objective lens labeled R19 and the position of the fieldstop during critical illumination, which is the surface labeled R14. Asmentioned above, this lens group is formed using two lens elements ofopposite refractive power with an on-axis spacing between the surfacesof 1 mm or less. An on-axis surface spacing of 1 mm or less in thisembodiment is achieved by having the two lens elements touching at theirperiphery and by carefully selecting the radii of curvature of the twosurfaces. On the other hand, if two lens elements used for chromaticaberration correction are set farther apart, a spacer would be required.This would cause the number of parts to increase, thereby increasing thecost of components as well as the cost of assembly. By making thecircumferential edges of the two lens elements contact each other, whileselecting the surface curvatures as defined in Table 2 above, an on-axisspacing of 0.0217 mm is achieved between the two lens elements ofopposite refractive power. Because this spacing is nearly that whichoccurs with a cemented lens, chromatic aberrations can be favorablycorrected just as in a cemented lens. In this way, high quality imagescan be provided without degradation of the optical components over timedue to ultraviolet excitation causing the cement of an achromatic,cemented doublet to become non-transparent with extended usage.

Further, the above Condition (4) is satisfied.

Embodiment 3

FIG. 3 shows the basic lens element configuration of an illuminationoptical system for a microscope according to a third embodiment of theinvention. In this embodiment, the single lens element illustrated withcross-hatching is designed to be moved along the optical axis in thedirection of the arrow when changing from Koehler illumination tocritical illumination.

Table 3 below lists the surface numbers #, in order, beginning with thelight source, the radius of curvature R (in mm) of each optical elementsurface, the on-axis spacing D (in mm) between surfaces, as well as theindex of refraction N (at 488 nm) and the Abbe number υ_(d) (at thed-line) for each lens element of Embodiment 3. In the middle portion ofthe table are listed the values for D9 and D11 for providing Koehlerversus critical illumination to the sample. In the bottom portion of thetable is listed the focal length of the illumination lens system of thisembodiment. In this embodiment, when the illumination state is Koehlerillumination, an image of the light source is formed at the aperturestop (R12). When the illumination state is critical illumination, animage of the light source is formed at the field stop (R13).

TABLE 3 # R D N υ_(d) 1 (Light source) 16.0010 2 19.5520 8.9300 1.4874970.21 3 13.3870 0.3000 4 −185.1800 6.0000 1.48749 70.21 5 34.4940 1.00006 −29.5740 8.0700 1.48749 70.21 7 83.2100 10.6300 8 18.8800 2.50001.59551 39.29 9 153.8230 D9 (variable) 10 −26.1226 12.0000 1.51633 64.1411 −67.7393 D11 (variable) 12 ∞ (aperture stop) 32.9900 13 ∞ (fieldstop) 30.9378 14 −559.9970 5.0000 1.51633 64.14 15 42.6387 62.9106 16 ∞(reflection surface) 30.0000 17 −72.5493 10.0000 1.51633 64.14 1836.8730 5.0000 1.59551 39.21 19 394.3018 196.5000 20 (OBJECTIVE pupil)for Koehler for critical illumination illumination Value of D9: 72.6534105.8781 Value of D11 69.7591  36.5344 Focal length of illumination lenssystem of this embodiment for Koehler illumination = −20.91 mm

As is evident from studying Table 3 above, the variable spacing D9 ischanged from 72.6534 mm to 105.8781 mm, and the variable spacing D11 ischanged from 69.7591 mm to 36.5344 mm when changing from Koehler tocritical illumination. This is achieved by moving the single lenselement of thickness D10 a distance 33.2247 mm toward the sample, asillustrated by the arrow.

In this case, the object optical system is an objective lens. Themagnification of the objective lens is 10× (ten times), so that thefocal length f₂ is 18.0 mm. The light source is an arc light sourcewhich is used in the microscope generally. The power of the arc lightsource is 100 watts, and the diameter “a” of the light emission regionis 0.6 mm. An image height is 22 mm so that the diameter “b” of thefield of view is 2.2 mm, and the ratio set forth above in Condition (1)has a value of 0.235, which satisfies Condition (1). Also, thisembodiment satisfies Condition (3).

As shown in FIG. 3, a field stop (the surface labeled R13) is arrangedat a position within the illumination lens system that is conjugate tothe position of the sample. Lens groups A and B, each of positiverefractive power, are positioned along the light path between the fieldstop and the pupil of the objective lens, with the lens group Apositioned on the light source side and the lens group B positioned onthe objective lens side. In addition, the above Condition (2) issatisfied, since f_(A) equals 76.955 mm and f_(B) equals 154.183 mm.

Further, a light reflection member (such as a mirror) is arrangedbetween the lens group A and the lens group B in order to fold theoptical path 900°. This prevents the position of the light source frombeing too remote from an operator. In addition, the lens group B iscomposed of a biconvex lens element and a lens element of negativerefractive power which jointly correct for chromatic aberrations. Thelens B is not positioned between two images of the light source whichare formed by Koehler illumination or by critical illumination,respectively. But the Abbe number υp of the positive lens element is64.14 and the Abbe number of the negative lens element υn is 39.21.Since the difference in these numbers is 24.93, Condition (4) above issatisfied.

Further, according to Embodiment 3, which is similar to Embodiment 1,the image of the light source is formed within the illumination lenssystem. This image is then relayed to the pupil of the objective lens,when the illumination state is Koehler illumination.

FIGS. 4A and 4B are schematic diagrams showing the basic configurationof optical components of the illumination optical system in the case ofutilizing Embodiment 3 to provide Koehler illumination to a sample. Inthis case, the illumination optical system is used for the invert typemicroscope. FIG. 4B is a partial schematic diagram of the components ofFIG. 4A as seen by a viewer looking in the direction indicated by thearrow from the position X in FIG. 4A. The components labeled as items1-9 are the same as in prior art FIGS. 7 and 8 and thus will not beseparately discussed. In FIG. 4A, there is an additional mirror 10 tofold the light path twice and a relay lens 11. By using a light paththat is folded twice, a more compact illumination optical system can beprovided, making it easier for the operator to reach the light source 1.

Embodiment 4

FIGS. 5A and 5B are schematic diagrams showing the basic configurationof optical components of the illumination optical system according to afourth embodiment of the present invention. FIG. 5B is a partialschematic diagram of the components of FIG. 5A as seen by a viewerlooking in the direction indicated by the arrow from position X in FIG.5A. The components labeled as items 1-9 are the same as in prior artFIGS. 7 and 8 and thus will not be separately discussed. In thisembodiment, the mirror 10 may be moved from a position where nomicroscope illumination light is incident to a position where light fromanother light source 1′ is incident onto the mirror 10 via a thirdillumination lens 11. The distance from light source 1′ to the mirror 10is different than the distance from light source 1 to mirror 10. Inaddition, mirror 10 may be rotated as indicated by the curved arrow toselectively provide one of two light sources 1, 1′ at differentdistances. This embodiment has an advantage in that the scope of themicroscope observation can be broadened. Thus, by rotating the mirror asindicated between the solid and dashed lines, the light from eitherlight source may be reflected to the sample. With this construction, therange of operability of the microscope can be enhanced.

Embodiment 5

FIGS. 6A and 6B are schematic diagrams showing the basic configurationof optical components of the illumination optical system according to afifth embodiment of the present invention. FIG. 6B is a partialschematic diagram of the components of FIG. 6A as seen by a viewerlooking in the direction indicated by the arrow from position X in FIG.6A. The components labeled as items 1-9 are the same as in prior artFIGS. 7 and 8 and thus will not be separately discussed. In thisembodiment, a dichroic mirror 13 is used as a reflection member for thepurpose of conveying light from a light source 1 onto the mirror 7. Inaddition, light from another light source 1′ is conveyed onto the mirror7 via the components 2 through 6, the mirror the dichroic mirror 13 andthe third illumination lens 11. Thus, in this embodiment, two lightsources 1 and 1′ can simultaneously illuminate a sample for the purposeof increasing the range of microscope observation. An alternativeconstruction would be to make the dichroic mirror 13 so as to be movableand rotatable for the purpose of enabling still another light source tobe used to illuminate the sample, similar to the situation illustratedfor Embodiment 4.

As mentioned above, according to the present invention, an illuminationoptical system for a microscope is provided wherein an appropriateillumination field and brightness can always be obtained for eitherKoehler illumination or critical illumination of a sample to be viewed.

The invention being thus described, it will be obvious that the same maybe varied in many ways. For example, a combination of Embodiments 4 and5 can be achieved by making the dichroic mirror 13 of Embodiment 5 bothmovable and rotatable while providing an additional light source. Suchvariations are not to be regarded as a departure from the spirit andscope of the invention. Rather, the scope of the invention shall bedefined as set forth in the following claims and their legalequivalents. All such modifications as would be obvious to one skilledin the art are intended to be included within the scope of the followingclaims.

1. In an illumination optical system for a microscope, comprising alight source, an illumination lens system that gathers light from saidlight source and directs the light toward a sample to be viewed, anobject optical system positioned between said illumination lens systemand said sample, the improvement of: making adjustable one of the lightsource position along the optical axis or the focal length of theillumination optical system so that illumination state changes fromKoehler illumination to critical illumination, respectively, thefollowing condition is satisfied:0.15<|(a·f ₂)/(b·f ₁)|<0.5 where a is the diameter of the light source,b is the diameter of the microscope field of view, f₁ is the focallength of the illumination lens system, and f₂ is the focal length ofthe object optical system; and wherein said lens group includes a lenselements having a positive refractive power and a lens element having anegative refractive power which, on axis, are touching or spaced in aira distance of 1 mm or less.
 2. The illumination optical system for amicroscope as set forth in claim 1, wherein a field stop is positionedwithin the illumination optical system conjugate to the sample surface,and the positive and negative lens elements have respective convex andconcave surfaces that touch, said positive and negative lens elementsbeing arranged between the field stop and the object lens.
 3. Theillumination optical system for a microscope as set forth in claim 1,wherein the positive and negative lens elements which, on axis, aretouching or spaced in air a distance of 1 mm or less satisfy thefollowing condition:15<υp−υn<50 where υp is the Abbe number of the positive lens element,and υn is the Abbe number of the negative lens element.
 4. In anillumination optical system for a microscope, comprising a light source,an illumination lens system that gathers light from said light sourceand directs the light toward a sample to be viewed, an object opticalsystem positioned between said illumination lens system and said sample,the improvement of: making adjustable one of the light source positionalong the optical axis or the focal length of the illumination opticalsystem so that the illumination state changes from Koehler illuminationto critical illumination, respectively, and the following condition issatisfied:0.15<|(a·f ₂)/(b·f ₁)|<0.5 where a is the diameter of the light source,b is the diameter of the microscope field of view, f₁ is the focallength of the illumination lens system, and f₂ is the focal length ofthe object optical system; and wherein said lens group includes a lenselement having a positive power and a lens element having a negativerefractive power which, on axis, are touching or spaced in air adistance of 0.3 mm or less.